Pneumatic tire are sometimes constructed with a circumferential belt reinforcement positioned in the crown portion between the tread and the carcass. Such construction is often used with radial ply carcass tires and sometimes with bias ply tires.
The belt is typically a fabric-reinforced rubber in the form of a rubber encapsulated textile fabric, or multiple filament, such as steel, glass, nylon, aramid or polyester. A steel fabric may be brass-coated in order to increase adhesion to rubber. The rubber of the belt compound may be and conventionally is natural rubber.
The belt reinforcement is used, for example, to control or limit the outward expansion or deformation of the tire in service, to provide some stability for the overall tire construction and to aid in providing a strengthening interface between the carcass plies and tread rubber.
Another and important purpose is to limit the distortion, or squirming, of the tread element as the tire rotates across a substrate (the ground or pavement, for example) and the tread passes through its footprint against the substrate. At this point of contact a considerable and continuing flexing and distortion of the tire is observed in the region of the footprint. Belt reinforcement resists such distortion. at the footprint which usually results in better tread wear characteristics. However, internal heat generation and temperature build-up within the tire and belt reinforcement as the tread distorts and moves through the footprint stresses the integrity of the belt reinforcement and may adversely affect its durability, particularly at the belt edges in the region of the tire sidewalls.
It is well recognized that the shoulder region of the tire undergoes a large amount of flexing and distortion when the tire is used in service, particularly under load. As a result, it is the shoulder region of the tire in which a substantial amount of heat build-up occurs, resulting in an increased tire running temperature in this region. Therefore heat durability of a tire reinforcing belt positioned between its tread and carcass, particularly at the belt edges in the shoulder region of the joining of tread and sidewall, is an important concern.
U.S. Pat. No. 4,483,381 to Richard M. Scriver (the inventor herein) shows and describes a pneumatic tire having a circumferential fabric-reinforced rubber belt positioned between the tread and the carcass wherein the rubber of the belt comprises about 10-45 weight percent medium vinyl polybutadiene rubber, about 55-90 weight percent cis-1,4-polyisoprene rubber (which may be either natural rubber, synthetic cis-1,4-polyisoprene, sometimes referred to as "synthetic natural rubber", or a blend of the two), and about 0-30% by weight cis-1,4-polybutadiene rubber. Data in the patent show that a blend of 40 parts of natural rubber, 40 parts of synthetic cis-1,4-polyisoprene and 20 parts (all by weight) of medium vinyl polybutadiene showed better adhesion to brass-coated steel cords aged at elevated temperatures in a nitrogen atmosphere, but poorer adhesion to cords aged at elevated temperatures in a water vapor atmosphere, than did a control composition comprising 50 parts by weight of natural rubber and 50 parts by weight of synthetic cis-1,4-polyisoprene.
U.S. Pat. No. 4,898,223 to Botzman et al is directed to a stiff rubber composition and the apex component of a tire made therefrom, wherein the stiff rubber composition is comprised of, based on 100 parts by weight rubber, a blend of (A) about 10 to about 60, preferably about 20 to about 40 parts by weight polyoctenamer rubber having a ML-4 viscosity value at 120.degree. C. in a range of about 4 to about 6 in its unvulcanized state, (B) about 10 to about 60, preferably 20 to about 40 parts by weight alkyl methacrylate grafted cis-1,4-polyisoprene rubber having a mole ratio of methacrylate to rubber in the range of about 0.25/1 to about 1.5/1, preferably about 0.4/1 to about 1/1, and (C) about 30 about 80 parts by weight of at least one additional sulfur curable rubber selected from natural rubber and synthetic rubber containing carbon-to-carbon double bonds therein. The polyoctenamer rubber acts principally as a processing aid. The rubbers of this patent are stiff, having a 3 point bend test value (ASTM D-4475-85) in excess of 20, and a relatively low elongation at break; a specific composition illustrated in the examples has an elongation at break of only 160 percent. Such rubber composition is useful as the rubber of the apex component of a tire since it enhances the tires cornering characteristic and consequently improves the cornering stability of a vehicle equipped with such tires.
U S. Pat. No. 4,824,899 to Yasuda furnishes another illustration of a stiff rubber suitable as the rubber of the apex component (which is called the "bead filler" in that patent). A metal acrylate is added to impart stiffness. Various specific compositions are disclosed, all of which have elongations at break of 200 percent or less. The disclosed purpose of the stiff apex or bead filler component is to improve cornering stability.
M. A. Wheelans, NR Technology, vol. 8, part 4 (1977), pp. 69-78 describes blends of natural rubber with "Heveaplus MG49" ranging from 30 to 80 parts by weight of natural rubber and conversely 20 to 70 parts by weight of "Heveaplus MG49". This article also describes vulcanizates of these blends and their use in energy absorption devices, particularly body components, (e.g., bumpers) in automobiles. The vulcanizates are elastomeric. "Heveaplus MG" is a tradename denoting a graft copolymer of natural rubber and methyl methacrylate (MMA) prepared by graft
polymerization of MMA in natural rubber latex according to NR Technology, vol. 12, part 4, 1981, pages 98 and 99. This graft copolymer can be used a reinforcing resin in natural rubber, according to NR Technology, vol. 8 (4), 1977, 70. Viscosity of the unvulcanized mix increases and elongation at break of the vulcanizate decreases, with increasing "Heveaplus MG49" content. "Heveaplus MG49" is available from plantation agencies SDN. Berhad Penang, Malaysia, according to the article. The later article NR Technology, 12 (4), 1981, pp. 98-99, discloses different grades of "Heveaplus MG", and nominal proportion of poly(methyl methacrylate) in "Heveaplus MG49" as 49% m/m.